2Physics Quote:
"Many of the molecules found by ROSINA DFMS in the coma of comet 67P are compatible with the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and in particular to the early Earth increasing drastically the concentration of life-related chemicals by impact on a closed water body. The fact that glycine was most probably formed on dust grains in the presolar stage also makes these molecules somehow universal, which means that what happened in the solar system could probably happen elsewhere in the Universe."
-- Kathrin Altwegg and the ROSINA Team
(Read Full Article:
"Glycine, an Amino Acid and Other Prebiotic Molecules in Comet 67P/Churyumov-Gerasimenko" )

Sunday, December 22, 2013

The Formation of Two Supermassive Black Holes from A Single Collapsing Supermassive Star

The existence of supermassive black holes with masses a billion times
the mass of our sun at high redshifts z>7 [1] is one of the mysteries
in our understanding of the early history of the universe. At
redshift z=7, the universe was less then one billion years old. This
leads to a serious problem: how is it possible for black holes to
acquire this tremendous amount of mass over a short timescale of just
one billion years? A common theory of black hole growth assumes as a
starting point the collapse of the very first stars, so called
Population III stars. Population III stars may have had masses around
100 times the mass of our sun. The collapse of such a star can leave
behind a black hole of similar mass that then grows via subsequent
accretion of material from its surroundings. This process can yield
quite massive black holes, but in order to reach supermassive scales
within only one billion years, the accretion process must be rather
extremely rapid to enable fast black hole growth. These high required
accretion rates, however, seem to be difficult to be maintained due
to, e.g. strong outgoing radiation that can blow away the surrounding
gas that otherwise would be accreted onto the black hole [2]. The
model, therefore, has difficulties of explaining the existence of very
massive black holes in the early universe.

Another model which has recently regained attention is supermassive
star collapse. Supermassive stars have originally been proposed by
Hoyle & Fowler in the 1960s as a model for strong distant radio
sources [3]. Such stars have masses up to a million times the mass of
our sun and potentially formed in the monolithic collapse of
primordial gas clouds that existed in the early universe [4, 5].
Unlike ordinary stars, which are mainly powered by nuclear burning,
supermassive stars are mainly stabilized against gravity by their own
photon radiation field that originates from the very high
interior temperatures generated by gravitational contraction.
During their short lives, they slowly cool due to the emitted photon radiation that keeps the stars in
hydrostatic equilibrium. The colder stellar gas can be more easily
compressed by the inward gravitational pull, and as a consequence, the
stars slowly contract and become more compact. This process continues
for a few million years until the stars reach sufficient compactness
for gravitationally instability to set in. This general relativistic
instability inevitably leads to gravitational collapse. One possible
outcome of the collapse is a massive black hole containing most of the
original mass of the star. Since the 'seed' mass of the nascent black
hole is already pretty large, subsequent growth via accretion from the
surroundings can easily push the black hole to supermassive scales
within the available time without the need of extreme accretion rates
and thus without any strong photon radiation that may blow away the
surrounding accreting gas.

In our recent article published in Physical Review Letters [6], we
study non-axisymmetric effects in the collapse of supermassive stars.
The starting point of our models are supermassive stars which are at
the onset of gravitational collapse. We use general relativistic
hydrodynamic supercomputer simulations with fully dynamical non-linear
space-time evolution to investigate the behavior and dynamics of
collapsing supermassive stars. Such computer models have been
considered in previous studies [7,8,9,10], however, mostly in
axisymmetry.

In an axisymmetric configuration, a supermassive star
maintains a spherical shape during its collapse, which is possibly
flattened due to rotation. In these previous studies, it has been
shown that the possible outcome is either a single massive rotating
black hole, or, alternatively, a powerful supernova explosion which
completely disrupts the star. In our case, we select an initial
stellar model which is rapidly rotating and leads to black hole
formation. In fact, it is so rapidly rotating that the shape of our
star is no longer spheroidal, but rather resembles the shape of a
'quasi'-torus where the maximum density is off-center and thus forming a
central high-density ring (see upper left panel of Figure 1).

Figure 1: (To view higher resolution click on the image) The various stages encountered during the collapse of a supermassive star with an initial m=2 standing density wave perturbation. Each panel shows the density distribution in the equatorial plane.

Such a configuration is unstable to tiny density perturbations that may be
present at the onset of collapse [10]. This instability is
particularly strong for perturbations in the form of standing poloidal
density waves with one (m=1) or two (m=2) maxima. Due to this
instability, these perturbations grow exponentially during the
collapse, and can lead to significant deformations away from
axisymmetry. The nature of the instability typically leads to the
formation of orbiting high-density clumps of matter inside the
collapsing star (see upper right panel of Figure 1). Since the m=1
and m=2 perturbations grow fastest, either one or two high-density
clumps will form, depending on the initial perturbation of the stellar
density. These high density fragments continue to grow rapidly during
the collapse, thus becoming denser and hotter.

Once temperatures of
more than one billion Kelvins are reached, a process sets, which is
called electron-positron pair creation. The creation of particle pairs
is possible because there is enough energy available in the
surrounding gas to spontaneously create a particle and its
anti-particle, in this case electrons and positrons. The pair creation
process has the effect of taking out energy from the gas fragments,
thus dramatically reducing their local pressure. The reduction in
pressure support leads to a rapid increase in the central density
within each fragment up to the point at which the fragments become so
dense that event horizons appear around each of them (center left
panel of Figure 1). In the case of an initial m=2 density
perturbation, two black holes form that orbit each other. Since two
black holes in close orbit emit very powerful gravitational radiation
- ripples of space-time that travel at the speed of light - , the
associated loss of energy causes the black hole orbits to shrink,
leading to an inspiralling motion (red lines in the center left panel
of Figure 1). The leading order mode of the corresponding emitted
gravitational wave signal is shown in the lower panel Figure 2.

Figure 2: (To view higher resolution click on the image)
The upper panel shows the time evolution of the density maximum until black hole formation. The center panel shows the mass and spin evolution of the black holes. The lower panel shows the emitted leading order gravitational wave signal.

It resembles the typical quasi-sinusoidal oscillatory signal expected
from binary black hole mergers: as the orbit shrinks, the emitted
radiation becomes higher in frequency. The inspiral continues until a
common event horizon appears, marking the merger of the two black
holes (lower left panel of Figure 1). The black hole merger remnant
is initially deformed into a peanut shape, which quickly relaxes into
a spherical shape by emitting exponentially decaying gravitational
ring-down radiation. This is shown in the lower panel of Figure
2. The peak amplitude of the waveform corresponds to the black hole
merger. From there, the signal quickly decays due to black hole
ring-down. By the end of our simulation, the remnant black hole is
rapidly rotating and is surrounded by a massive accretion disk (lower
right panel of Figure 1).

The formation of two merging black holes requires a particular choice
of initial stellar model parameters at the onset of collapse: (i) we
require rapid rotation and (ii) a poloidal m=2 standing density wave
perturbation must be present. This naturally leads to the question of
the likelihood of our model. Recent cosmological simulations of
collapsing primordial gas clouds - the potential birth sites for
supermassive stars - indicate that rapid rotation is very likely
[4]. Curiously, the same simulations also show that an m=2 deformation
arises at the center of the clouds where the supermassive star will
eventually form. Unfortunately, these simulations currently do not
offer sufficient spatial resolution to investigate the formation of
supermassive stars in the collapse of primordial gas clouds in detail.
Further research will be necessary to self-consistently model the
formation of supermassive stars that may inform us about the stellar
conditions at the onset of collapse.

The new and exciting prediction that two black holes can form in the
collapse of a single star gives rise to very efficient gravitational
wave emission compared to models where only one black hole forms. The
emitted gravitational radiation in our model configuration is so
powerful that future space-borne gravitational wave observatories
might see the signal from the edge of our universe. This has
important implications for cosmology. If detected, the signal will
inform us about the formation processes of supermassive stars and
supermassive black holes in the early universe and will allow us to
test the validity of the supermassive star collapse pathway to
supermassive black hole formation.

Acknowledgements: This research is partially
supported by NSF grant nos. PHY-1151197, AST-1212170, PHY-1212460,
and OCI-0905046, by the Alfred P. Sloan Foundation, and by the Sherman
Fairchild Foundation. CR acknowledges support by NASA through
Einstein Postdoctoral Fellowship grant number PF2-130099 awarded by
the Chandra X-ray center, which is operated by the Smithsonian
Astrophysical Observatory for NASA under contract NAS8-03060. RH
acknowledges support by the Natural Sciences and Engineering Council
of Canada. The simulations were performed on the Caltech compute
cluster Zwicky (NSF MRI award No. PHY-0960291), on
supercomputers of the NSF XSEDE network under computer time allocation
TG-PHY100033, on machines of the Louisiana Optical Network Initiative
under grant loni_numrel08, and at the National Energy Research
Scientific Computing Center (NERSC), which is supported by the Office
of Science of the US Department of Energy under contract
DE-AC02-05CH11231.

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